Control of coke particle size in fluid coking process



Dec. 23, 1958 c. E. JAHNIG ET AL 2,865,847

CONTROL oF coxE PARTICLE SIZE 1N FLUID coxING PROCESS Filed Aug. 3, 1953CHARLES E. JAHNIG WALTER G. MAY iNVENTORS BERNARD I .SC ULMAN BY /ffw)m/AH ATToRNEY vsive size reduction in the grinder.

United States Patent F CONTRDL F COKE PARTICLE SIZE IN FLUID COKINGPROCESS Charles E. Jahnig, Red Bank, Walter G. May, Union, and BernardL. Schulman, Roselle, N. J., assignors to Esso Research and EngineeringCompany, a corporation of Delaware Application August 3, 1953, SerialNo. 371,798

4 Claims. (Cl. 208-152) This invention relates to the art of cokingheavy hydrocarbons, and particularly to an improvement in controllingcoke particle size in the coking reactor. More specifically it relatesto a coking process and apparatus wherein properly sized seed coke issupplied to the coking reactor by selectively burning and grindingcoarse coke and selectively returning the relatively line portion of theground coke tothe reactor.

Processes for coking heavy hydrocarbon oils in contact with a fluidizedbed of iinely divided and essentially inert contact solids such as cokeor sand are basically well known. However, in applying such processes ona commercial scale, numerous diiiculties are encountered. For instance,as more and more feed is deposited and coked on the contact solids, thelatter continuously grow in size until they become too large to beproperly uidized. Also, it is found that with larger particles theallowable feed rate relative to the total weight of coke present isreduced because of eventual sticking of the particles in the bed.

Consequently, it is desirable to maintain the average particle size inthe reactor substantially constant. This can be accomplished by feedingin small seed coke particles in substantially the same numbers as largecoke particles are withdrawn from the process. But here again anotherproblem is encountered in connection with the loss of the desired finesfrom the system. Normally the heat requirement of fluid coking processesis supplied by withdrawing a part of the coke from the coking reactor,partially burning the withdrawn coke, and returning the reheatedremainder to the coking reactor. However, combustion tends to consume alarge portion of any lines present and a further amount of coke fines isusually lost in the line gas when the latter is separated from thereheated coke. preparing additional line seed coke may add greatly tothe cost of the` entire process. Furthermore, the loss of fines throughthe Hue stack may be most undesirable not only for the aforementionedoperational and economic reasons, but also because of the resulting airpollution.

It is an object of the present invention to provide a coking processwherein particle size distribution in the coking reactor may becontrolled within. desirable limits and lluidization diiculties due ltooversize particles avoided. Another object is to facilitate theselective withdrawal of large product particles from the reactor whilesupplying thereto the necessary amount of fine seed particles. The largeparticles may then be circulated through a heating system, from whichthe tine seeds are excluded to avoid losing them. A further object is toreduce the amount of grinding necessary for the production of the seedparticles -by selective partial combustion of the coarse coke particlesso that they are reduced in size. Anotherobject is to improve grindingselectivity by combining elutriation and grinding to prevent exces-Still another object is to accomplish the foregoing simultaneously witha As a result the cost of r V2,865,847 Patented Dec. 23, 1958 ICCminimum loss of fines in the flue gases leaving the process. These andother objects as well as the nature and advantages of the presentinvention will become more clearly apparent from the followingdescription, especially when read in connection with the accompanyingdrawlng.

Figure l of the drawing is a semi-diagrammatic illustration of aspecific embodiment of the invention wherein a petroleum pitch is cokedin contact with a dense uidized mass of finely divided coke, and

Fig. 2 shows an alternate arrangement wherein the elutriatorcharacteristic of the present invention is immediately beneath the mainreactor. Referring to Fig. l of the drawing, petroleum pitch having agravity of about 10 to 20 API, e. g., 5 API, an initial atmosphericboiling point of about 900-1200" F., e. g., 1l00 F., and a Conradsoncarbon content of about 5-50 weight percent, e. g. 25%, is preheated byconventional means, not shown, to about SOO-900 F., e. g., 700 F., andthen introduced or sprayed through line 1 into coking vessel 10. Thoughnot essential, the feed may be mixed with say 1 to 10 weight percentsteam to disperse it as it is introduced into the reactor. The cokingvessel l0 contains coke particles ranging in size from about 50 to 1G00microns, e. g., to 500 microns, which are maintained in the form of adense turbulent mass 11 having an upper level 12 above which is a dilutephase 13. Obviously, a small amount of particles or particle aggregatesmay form in the course of the process which are much larger than thesize just indicated, and lumps as large as one inch may occasionally beencountered. The coke particles are maintained liuidized by theupflowing hydrocarbon vapors formed by the coking of the pitch and alsoby steam which is introduced into the lower part of vessel 10 throughline 21.

This steam addition rate is adjusted so as to provide together with thehydrocarbon vapors a total superficial upward gas velocity of about 0.5to 5 ft./sec.,. e. g., 3 ft./sec. The density of the fluidized coke inbed 11 may thus be between about 30 and 60 lbs/cu. ft., e. g., 40lbs/cu. ft., while the temperature of bed 1l is maintained at about 850to ll00 F., e. g. at 950 F. The pressure in the upper part of cokingvessel 10 is high enough to overcome the pressure drop through therecovery equipment in which the vaporized hydrocarbon products must befractionated or otherwise treated after their withdrawal from cokingreactor l0. For instance, the pressure in the dilute phase 13 may be ofthe order of about l to l0 p. s. i. g., though considerably higherpressures such as p. s. i. g., or on the other hand, subatmosphericpressure, may be preferred under special circumstances. At the bottom ofreactor 10 the pressure is, of course, considerably higher than at thetop of the reactor due to the hydrostatic head exerted by the densefluidized solids. Such a hydrostatichead may amount to about l0 to 20pounds per square inch in equipment of commercial size.

Vaporous products of coking pass overhead from uidized bed 1l. Thesevapors contain some entrained solids and form the dilute phase i3. Thevapors are passed through gas-solids separating means 14 such as one ormore cyclones which separate entraine'd solids and return them toiluidized bed 11 through dip pipe 16. The more or less dust-free vaporsthen pass overhead through line 15 for further treatment in equipmentwhich may include a fractionating tower, a catalytic cracking unit, andother conventional apparatus which need not be illustrated.

As the hydrocarbon feed is coked in vessel l0, it is endothermallydecomposed into hydrocarbon vapors as well as a solid carbonaceousresidue. This -solid residue deposits in tilmlike layers on the finelydivided fluidized particles, onto which.the liquid hydrocarbon feed issprayed, causing a continuous growth of the particles. As was explainedbefore, this has an adverse eftect on the continued operation', ofthecolringV step. It'. has,

therefore,been foundgadvantageousto maintain a more or less constantparticlesize'distribution inthe colringr bed by continuously; or atleast periodicallyv removingy relatively coarse particles; andreplacing; themt by rela-vV ortunately, combustion tends to consume thevery tine,

particles, leaving armixture of particles having; an average particlesize only somewhat smaller'thanytheaverage size of the particles fedinto the combutiojn zone.

Moreover, the colte production. rate is sometimes muchl greater thanthat needed for fuel in the heater. For

instance, at comparatively low coking temperaturesand. with high recyclerates to the coker, the coke formed in the process may be 59%.more thanthe'Conradson.-

carbon content` ofthe feed. Hence, while such combustion reducessomewhat the average particle size of the withdrawn coke, it is not veryeffe tive by itself for supplying the needed seed coke. The latter maybe of any size smaller than that of the product coke, but the amountrequired decreases rapidly with size. Therefore, the seed normally willbe in the size range of about '5U-15G microns, and preferably shouldhave a diameter not greater than 1/2 and preferably equal to 1/3 or Mior less of the diameter of the product coke withdrawn, since the amountof seed coke required decreases rapidly as the size of the seed cokeparticles is decreased.

In accordance with the present invention specic steps are taken toassure that only coke consisting largely of relatively coarse particlesis withdrawn from the reaction zone for combustion or product recoveryand that only a fraction relatively rich in fine particles is returnedto thereaction zone. A portion of the withdrawn coarse colte is heatedas well as reduced in size by combustion in a combustion zone whileadditional comminution is obtained by grinding another portion of therelatively coarse coke toobtain the required amountof nely di. videdseed coke.

pable of separating the withdrawn coke into relatively line particlesuseful as seed coke, and relatively coarse solids. A portion of thecoarse solids is then passed to a heater while anotherportion is groundto bring the supply of seed coke to the required amount; and any surplusof the coarser coke may he recovered as product.

Referring specifically again to Fig. l of the draw-ing, the coke may bewithdrawn from bed l1 through line 19 into anv intermediate portion of adilute phase elutriator 2i) where it is met by an upowing current of gassuch as steam introduced through line 27. This gas is preferablyintroduced somewhat above the bottomrof' the vessel, that is, above thedense phase of coarse solids which forms at the bottom of the elutriatorand whence the coarsevsolids are withdrawn. The elutriator preferablyalso contains several perforated transverse battles 26 which serve tobreak up any localized streamers and thus increase the etiiciency of theseparation. When gas is passed upwardly through such an elutriator at aproper velocity and a mixture of solid particles of different sizes issprayed or otherwise evenly introducedacross an intermediate portionofthe elutriator, relatively coarse particles fall to the bottom while neparticles are entrained overhead and pass along with the elutriating gasthrough linel back to reaction zone 11. ln this way line par- Thus, cokemay be withdrawn from. the uidized coking bed and passed to anelutriator caticles capable of serving as seed coke are retained in orcontinuously recycled to the coking zone.

The two principal variables affecting such separation of coarse and finesolids are the elutriant gas velocity and the rate at which the mixtureof particles is fed into the column. In order to remove a major portionof the fines from the coarse, the elutriant gas velocity should be atleast 1.5 to 3 times, e. g., about twice the free fall velocity of thelargest particle to be taken overhead. As the gas velocity is increasedless of the fines fall to the bottom with the coarse material. However,at the same time, as the gas velocityy exceeds the free fall velocity ofany particles of the coarse fraction, some of these coarse particleswill be carried overhead and contaminate the lines. For practicalpurposes the elutriant `gas velocity may range from about 3 ft./sec. ifit is desired to recover lines of about micron diameter and liner, toabout 3() ft./sec. if solids to be carried overhead are to includeparticles of about 1000 micron diameter. Of course, the optimumelutriant velocity will vary somewhat from case to case depending on theparticle sizes which one may wish to separate, the desired yield oflines recovery, the permissible contaminationl of lines with coarsermaterial, and the solids feed rate. Likewise, with comparatively small,under-designed elutria tors relatively higher velocities may bepreferred so as to prevent an excessive amount of fines from going downwith. the coarser material.

The solids feed rate to the elutriator also has a pronounced effect onthe degree of separation. For a given gas velocity, as the feed rateincreases the amount of fines going to the bottom increases. lf there isany amountA of coarse materia-l going overhead because the gas velocityexceeds the free fall velocity of the lines actually desired, anincrease in the solids feed rate will decrease the amount of coarsematerial in the overhead.

At the same time the total amount of material goingv overhead decreasesin proportion to the amount dropping to the bottom. Consequently, thereis an optimum ratio of solids feed rate to gas rate for cach gasvelocity, which will give only a small amount of coarse material in theoverhead and only a small amount of lines in the bottom.

For instance, to separate approximately 250 micron and smaller cokeparticles (free fall velocity about 4 ft./sec.) from coarser cokeparticles, Table I represents the best range of solids feed'rate to gasrate for each given gas velocity.

TABLE I Gas velocity Feed rate+gas rate.

4 ft./s`ec QCS-0.075 lbs/cla ft. 5 0.075-0.l25.

lf the ratio of feed rate to gas rate is less than that shown above, agreater amount of coarse material is taken overhead and the capacity ofthe column is decreased. Conversely, when ratios greater than thoseshown above are used, the amount of nes lost to the bottom becomes high.This is illustrated in T able ll.

The described combination of a dilute phase elutriator and a uid cokeris particularly effective since it permits using the gas in theelutriator both as an elutriation gas and as a stripping gas forremoving volatilizable hydrocarbons from the coke withdrawn from thereaction zone. However, instead of employing a dilute phase elutriatorof the type described, the elutriator may contain very coarse packingsuch as large Raschig rings or the like. In addition, for taking outparticles much larger than average circulating coke, e. g. aggregates of0.5 inch diameter or larger, dense phase separation may be used.

The coarse solids which concentrate in the bottom of elutriator arewithdrawn through line 22. The with- 1 drawn coarse solids, most ofwhich may range in size from about 200 to 800 microns, with some largerparticles or agglomerates and some smaller particles or fines, may beseparated into three portions. One portion constituting the net cokeproduced in the process may be withdrawn through line and, aftersuitable cooling with a water spray or the like, passed to storage. Thisproduct coke may amount to about 10 to 35 weight percent, e. g., 25%,based on residual hydrocarbon feed to the reactor, and may nd use as afuel, as metallurgical coke, etc.

Another portion of the coarse coke which may equal about 5 to 15 times,e. g., about 10 times the weight of residual hydrocarbon feed, isrecirculated through a heating zone to supply heat to the reactor 10.Accordingly, coke from line 22 is passed through line 24, suspended inan oxygen-containing gas such as air injected through line 3i, and theresulting dilute suspension then passed upwardly through a heater whichmay be an upflow burner where the coke settles out of the suspension toform a conventional dense, fluidized bed. In burner 30 a portion of thecoke is consumed while the remainder is heated to a temperature about 25to 300 F. higher than the coking temperature, e. g. to 1250 to l500 F.Alternatively the coke may be heated in a high velocity transfer lineburner of an otherwise conventional type. In either case, hot coke fromthe burner may be passed through a dust separating device such ascyclone 34, from which the separated combustion gases may be withdrawnthrough stack while the separated reheated coke is returned to thecoking zone 1i through lines 37 and 38, to supply the heat requirementsof the coking reactor. Some hot coke may also be' passed from burnerstandpipe 37 directly to the elutriator-stripper 20 through line 33, topermit keeping the temperature in vessel 20 at an optimum level forstripping.

Where coke commands a premium over fuels such as gaseous or liquidhydrocarbons, a suitable extraneous fuel may be injected into the burnersystem through line 32. In such a case the circulating coke is reheatedprimarily by contact with the hot gases produced by the combustion ofthe extraneous fuel. Furthermore, it will of course be understood thatthe heat requirement of the process may be satlstied by still othermeans, e. g., by means of hot flue gases obtained in an independentlytired auxiliary burner, or by indirectly reheating the coke in anotherwise well known manner while maintaining it as a densely uidizedbed, etc.

The third portion of coarse coke withdrawn from the elutriator is passedthrough line 23 to grinder 40. This last portion is a comparativelysmall one, equalling normally only about 10 to 15 weight percent onresidual feed, or only about 1% of the coke passing from the reactor butnonetheless it represents one of the critical features of the invention.Preferably, though not necessarily, this stream of coke is cooled to amoderate temperature, say to about 100 to 250 F., before being actuallyintroduced into the grinder. This cooling may be done in any convenientmanner, e. g., by passing the coke in an aerated condition through aheat exchanger vessel 50 which may contain a watercooled coil 5l.Grinder 40 may be of the ball or rod mill type, or other kinds ofgrinders suitable for the handling of coke, such as hammer mills, rollermills, or -conventional gas jet grinding, may be used likewise. Ingrinder 40 the coarse coke is comminuted to produce a substantialfractionl of particles smaller than at least about 200 microns. Grindingmust be sufficient to provide the seeds needed to control particle sizein the coker. This required amount of f which may range in sizepredominantly between about 200 and 500 microns, may be ground ingrinder 40 until about 30 to 50 weight percent of the ground cokemixture is smaller than microns in order to supply the fine particlesdesired as seed coke for return to the coking reactor.

However, since such seed coke is preferably a narrow cut of roughly 50to 150 micron size, in previously contemplated coking processes thisrequired screening of the ground material to provide the desired sizefraction. In accordance with the present invention this screening stepis eliminated by returning coke from the grinder 40 to elutriator 20 asshown by lines 41 and 42 or 43. Return of the ground coke to theelutriator may be accomplished by lifting the coke as a suspension withthe aid of steam injected through line 44. If desired, the steam may beseparated in cyclone 45 and eventually introduced into the bottom of theelutriator through line 27. Lines 42 or 43 may be used in thealternative, depending on whether it is considered important to keep thedesired line particles out of the heavy tailings as much as possible, inwhich case the coke is fed back through the upper line 43, or whether itis preferred to utilize the gas added through line 44 for elutriation inVessel 20, in which case the coke is preferably fed back through lowerline 42. The choice is also affected by the diameter ratio of seed toproduct coke. With smaller seeds, the amount required is decreased, andthe gas quantity to line 44 is less so that it is of less importance andcan be added at the higher point through line 4 3. Conversely, withlarge amounts of coke circulating around the elutriator, that is, whenusing relatively coarse coke as seed, it may be preferable to feed theground coke suspension through line 42 so as to utilize the large amountof gas present.

It will be seen that in this invention the same gas stream can be usedfor elutriation of product coke, air classification of ground coke,stripping of spent coke, and aeration of the coker reactor. This reducesthe consumption of steam or other fluidizing gas and decreases the loadon the reactor and product handling equipment.

Referring to Fig. 2 of the drawing, this shows an alternate arrangementwherein the elutriator is directly beneath the reactor vessel. from thewide reactor vessel 10 pass through line 19 into the narrowerelutriating section 20 which is directly beneath the reactor and whencethe fines are blown back into the reactor. The coarser particles areagain withdrawn downwardly from the elutriator through line v22, thesurplus to be recovered as product, while the remainder is burned orground to finer size as needed similarly as in Fig. 1. The grindingsection, which again may include a cooler 50 and a cyclone 45 as in Fig.1, is shown in greatly oversimpliied form in Fig. 2. The groundparticles from grinder 40 are again returned to elutriator 20 and thenceto the reactor as previously described. Eflicient use of the coke isagain assured due to the closed-cycle grinding obtainable when groundcoke isl passed through an elutriator for separation of coarse and tineparticles and the separated coarse particles are recycled to thegrinder.

Example The advantages of the present invention of elutriating inconnection with seed coke grinding in iiuid coking processes are furtherillustrated by the following cases derived for a tiuid coking unithaving a petroleum pitch feed rate of 23,000 barrels per day and havinga net coke make of 10 weight percent on feed, or 400 tons per In thisdesign the solidsv 7, day, operating: on a scheme substantiallyasillustrated in-Fig.- 1. The data are summarized inTable III.

TABLE III COKE GRINDING REQUIREMENTS Case No 1 2 3 4 Seed Coke Size',microns; 110 110 55 55 Average Circulating Coke Size,

microns 195 195 195 195 Elutriation Perfect None Perfect None CokeWithdrawn (To grinder and as product):

Tons/day 434 490 403 409 Average' size, microns 258 195 284 195 SeedReqd., Tons/day.. 34r 9D 3 9 Grinding Power, H; P; 22 45 6 l5 Size-Distribution vof Circulating Coke:

Percentonmesh .(833 micron) 1 35mesh (417 micron) 30 4S mesh (295micron) 0 58 60 mesh (246 micron) 18 70 80 mesh (175 micron) S2 87 100mesh (147 micron) 92 92 150 mesh (104 micr0n) 100 100 98 97 200 mesh (74micron 99 325 mesh (43 micron)v. 100

In the perfect elutriation cases l and 3 all particles larger'thanspecified size drop down and are withdrawn from the-bottom, whileparticles of the specified size and v are entrained overhead intheelutriator and returned to thereactor as seed coke, and only coarserparticles are passed to the grinder or withdrawn as` product. Incontrast, in case 2 the elutriator is completely by-passed soy that cokefrom the reactor is. passed directly to the grinder and the ground cokemixture-is returned from the grinder directly to the reactor. case lythe relatively coarse particles are selectively ground whereas in casev2 a considerable amount of coarse materialcirculates in the system. As aresult of this preferential grindingof coarse particles, andpreferential return offine particles to the reactor,'the.weight of seedsr required as .well as the correspondinggrinding power are verymuchsmaller in case l than in case 2.

Similar conclusions can' also be derived from a comparisonY of cases 3and 4,-where the coke was ground to a much finer size. In addition, avcomparison of cases' l and 3, or 2 and 4, shows that as the seed cokesizeis' reduced by a factor of about 2, the amount of seed coke requiredis very greatly reduced, approximately by a factor of 10. Thisis due tothe fact that the amount ofy seed` coke required is approximatelyproportional to the cube of the average particle diameter. In otherwords, since the main purpose ofthe seed coke is to keep `the averageparticle size constant by supplying a certainl number of relativelysmall particles, it will be apparentthat this number of particles in agiven total weight;increases rapidly as the average particle size' isdiminished. However, excessively small seed is not practical in view ofthe attendant difiiculty of keeping such fine particles from blowing outof the system. Thus, while' 110 micron size particles can be fairlyefiiciently recoveredin a vsingle separator cyclone, 55 micron particlesnormallyv will require a two-stage cyclone, and stillsmaller particleswill be still more difficult to recover` It can be noted from. theforegoing description that the present .invention permits unusuallyefficient use of Consequently, in`

coke since the fines suitable as seed coke are selectively retained inthe reactor or immediately returned to it but are not allowed to pass tothe burner, or be withdrawn with the product coke. Furthermore,selective combustion of the coarse particles in the burner reduces theamount of grinding otherwise required to maintain the proper particlesize distribution.

It will be understood, of course, that the foregoing general descriptionand specific embodiment of the inventionas applied to fluid coking hasbeen given principally for purposes of illustration rather thanlimitation. On the contrary, the invention can be still further modifiedin various ways without departing from the scope or spirit hereof. Thus,since the amount of coke circulated to burner 30l through line 24 isusually relatively large, it may requirea large amount of gas tocompletely elutriate the entire stream* If desired, therefore, the cokestream to theburner may be withdrawn separately from above the bottom ofthe elutriator, and the bottom section used for more completelyelutriating the coke stream in a second stage for seed grinding. Gasfrom the latter then is used in the upperl zone which elutrates the coketo the burner, or the coke passing from the reactor to the burner may beallowed'to by-pass the elutriator altogether, and only coke to and fromthe grinder may be elutriated to provide seed coke and, if desired,coarse product.

The invention is particularly pointed out in the appended claims.

We claim:

l. In a process for colring heavy hydrocarbonaceous material wherein thehydrocarbonaceous material is fed into a dense turbulent fluidized bedof finely divided essentially inert refractory solids maintained in acoking zone at coking temperature to produce lower boiling hydrocarbo-nvapors which are removed overhead and a coke-like deposit which remainson the fluidized solids, and the coke-containing solids are withdrawnfrom said uiclized bed, heated to a temperature of at least l000 F, andreturned'to saidfcolring zone to maintain said fluidized bed at cokingtemperature, the improvement which comprises passing the solidswithdrawn from said coking zone to a dilute phase elutriation Zone,passing an inert gas upwardly through the solids in a dilute phase inthe elutriation zone at a velocity of'at least 1.5 to 3 times the freefall velocity of the largest particles to be entrained, sufficient toentrain particles smaller than about 175 microns, removing the inert gasand eutrained particles upwardly from the elutriation zone and passingthem directly into the aforesaid liuidized bed in said coking zone,withdrawing relatively coarse particles downwardly from the elutriationzone, grinding the relatively coarse particles, and returning the groundparticles to the aforesaid elutriation zone` 2. in a process for cokingheavy hydrocarbonaceous material wherein the hydrocarbonaceous materialis fed into a dense turbulent fluidized bed of finely dividedessentially inert refractory solids maintained in a coldng zone atcoking temperature to produce lower boiling hydro-carbon vapors whichare removed overhead and a coke-like deposit which remains on thefluidized solids, and the coke-containing solids are withdrawn from saidiluidized bed, heated to a temperature of at least 1000 F. and returnedto said coking zone to maintain said fluidized bed at cokingtemperature, the improvement which 4comprises passing the solidswithdrawn from said coking zone to an intermediate portion of a verticaldilute phase elutriation zone, passing an inert gas upwardly through thedilute phase solids in the elutriation zone at a velocity of at leastabout 3 feet per second so as to entrain substantially selectivelyparticles smaller than about microns, removing the inert gas andentrained -particles upwardly from the elutriation zone and passing themdirectly into the bottom portion of the aforesaid iluidized bed in saidcoking zone, withdrawing relatively coarse particles downwardly from theelutriation zone, passing a major portion of said withdrawn relativelycoarse particles through a heating zone where the last-named particlesare partially burned and heated, returning the heated particles to saidcoking zone, passing a minor portion of said withdrawn relatively coarseparticles to a grinding zone where the said coarse particles are groundto produce a substantial fraction of particles ranging in size betweenabout 50 and 150 microns, and returning said ground particles from saidgrinding zone to said elutriation zone.

3, A process according to claim 2 which comprises the additionalspecific steps of suspending said ground particles withdrawn from thegrinding zone in an inert lift gas, passing the resulting dilutesuspension to an external point elevated above an intermediate level ofthe elutriation zone, separating the lift gas from said groundparticles, passing the separated ground particles into said elutriationzone at said intermediate level, and externally passing said separatedlift gas into a bottom portion of said elutriation zone to serve aselutriation gas.

4. A process according to claim 2lwherein said solids withdrawn from thecoking zone are passed to a lrst elutriation stage, elutriating gas ispassed upwardly through said ir'st elutriation stage to eifect a roughseparation of the solids into relatively large and relatively fineparticles, said separated relatively ne particles and elutriating gasare returned directly to the bottom of the aforesaid coking zone, amajor portion of said separated relatively large particles is passed tothe aforesaid heat ing zone, a minor portion of said separatedrelatively large particles is passed downwardly to a second elutriationstage for further separation, elutriation gas is passed upwardly throughsaid second elutriation stage to separate the last-named relativelylarge particles into relatively coarse and additional relatively neparticles, said elutriation gas and entrained fine particles are passedupwardly to the bottom portion of said first elutriation stage, at leasta portion of said separated relatively coarse particles is passed fromsaid second elutriation stage to a grinding zone where said coarseparticles are ground, and the resulting mixture of ground particles isreturned to an intermediate portion of said second elutriation stage.

References Cited in the file of this patent UNITED STATES PATENTS2,661,324 Leffer Dec. l, 1953 2,707,702 Watson May 3, 1955 2,721,168Kimberlin et al Oct. 18, 1955

1. IN A PROCESS FOR COKING HEAVY HYDROCARBINACEOUS MATERIAL WHEREIN THEHYDROCARBONACEOUS MATERIAL IS FED INTO A DENSE TURBULENT FLUIDIZED BEDOF FINELY DIVIDED ESSENTIALLY INERT REFRACTORY SOLIDS MAINTAINED IN ACOKING ZONE AT COKING TEMPERATURE TO PRODUCE LOWER BOILING HYDROCARBONVAPORS WHICH ARE REMOVED OVERHEAD AND A COKE-LIKE DEPOSIT WHICH REMAINSON THE FLUIDIZED SOLIDS, AND THE COKE-CONTAINING SOLIDS ARE WITHDRAWNFROM SAID FLUIDIZED BED, HEATED TO A TEMPERATURE OF AT LEAST 1000*F. ANDRETURNED TO SAID COKING ZONE TO MAINTAIN SAID FLUIDIZED BED AT COKINGTEMPERATURE, THE IMPROVEMENT WHICH COMPRISES PASSING THE SOLIDSWITHDRAWN FROM SAID COKING ZONE TO A DILUTE PHASE ELUTRIATION ZONE,PASSING AN INERT GAS UPWARDLY THROUGH THE SOLIDS IN A DILUTE PHASE INTHE ELUTRIATION ZONE AT A VELOCITY OF AT LEAST 1.5 TO 3 TIMES THE FREEFALL VELOCITY OF THE LARGEST PARTICLES TO BE ENTRAINED, SUFFICIENT TOENTRAIN PARTICLES SMALLER THAN ABOUT 175 MICRONS REMOVING THE INERT GASAND ENTRAINED PARTICLES UPWARDLY FROM THE ELUTRIATION ZONE AND PASSINGTHEM DIRECTLY INTO THE AFORESAID FLUIDIZED BED IN SAID COKING ZONE,WITHDRAWING RELATIVELY COARSE PARTICLES DOWNWARDLY FROM THE ELUTRIATIONZONE, GRINDING THE RELATIVELY COARSE PARTICLES, AND RETURNING THE GROUNDPARTICLES TO THE AFORESAID ELUTRIATION ZONE.